Historically, phenolic resins have been the preferred thermosetting resin material when low smoke emission and self-extinguishing ability are of paramount importance in a fire situation. Applications are in building, heating, ventilation, and pipe insulation where phenolic foams provide both thermal insulation and fire resistance.
Presently, in phenolic cellular foam manufacture, a phenolic resin such as a resole resin is commonly catalysed by either a strong organic or inorganic acid. For example, EP 0 170 357A describes a process for the production of an acid cured phenolic resin foam. The selection of acid type is dependent on the desired curing time and temperature. Cellular insulation foam is produced when the blowing agent that has been blended into the resin starts to boil. Hydrocarbons or mixtures thereof are commonly used blowing agents. Expansion typically occurs in the temperature range 15° C. to 70° C. Care needs to be taken in the manufacture of phenolic foam to ensure that an excessive resin exotherm does not develop. If the total % water content in the formulated resin system is too low, the occurrence of an uncontrolled exothermic chemical reaction is more likely for example when a strong acid is used as catalyst. When exothermic reactions develop, further amounts of water or steam are created by the phenolic resin condensation polymerisation reaction. This adversely affects the ability to form closed cell foam. Similarly, if the selected phenolic resin has too high a water content, closed cell foam will not be produced. Closed cell foam structure is highly desirable to maximise insulation performance. By controlling the % water content of the uncured phenolic resin system, prior to activating curing and the blowing agent, and curing at elevated temperatures it is possible to produce phenolic foam that has a closed cell structure. It is also necessary to select the type and amount of each of the phenolic resin; acid catalyst; surfactant and blowing agent, for mixing and curing at elevated temperature to achieve the desired closed cell foam.
Electron microscopy can be used to demonstrate whether foam cells have defects such as holes or cracks. It is desirable to have low density, defect free, closed cell foam for low cost, stable thermal insulation. Defects in cells can lead to a loss of chemical blowing agent from the cells and air diffusing into the cells raising thermal conductivity. This is undesirable for an insulation material.
In particular, there is a need to provide low-density closed cell phenolic foams without holes or cracks in the cells. Further, there is a need for a phenolic resin system that can be easily mixed at moderate temperatures (10 to 30° C.). Low viscosity resin systems are preferred for ease of mixing in manufacturing on a commercial basis.
Phenolic foam can be prepared in blocks, laminated boards or as moulded sections of a particular shape. In one industrial process, laminated phenolic foam insulation boards are manufactured with typical thickness 20 mm to 150 mm and a dry density of 25 to 60 kg/m3. In this process, phenolic resin, acid, and blowing agent are mixed using a conventional high shear or high pressure mixer head. The catalysed liquid resin is then introduced into a foam laminating machine and progressed between aluminium foil, steel plates, paper or glass mat facings. Foaming and curing then commences and the resin cures to form a foam product. These foam products, that include for example insulation boards, are typically produced at 50 to 80° C. in about 2 to 15 minutes. The foam products then often require further cure, typically at elevated temperatures such as by what is often referred to as an oven “post cure”, for example at 50 to 90° C., for 1 to 72 hours, to develop sufficient handling strength. The resin system typically comprises the following chemical ingredients listed with typical weight proportions parts by weight (pbw):                Liquid phenolic resole resin (typically 60-85% cured solids) containing 1 to 10% surfactant: 100 pbw        Blowing agent (typically halocarbon and/or hydrocarbon based): 4-20 pbw        Strong organic or mineral acid 9-30 pbw.        
When phenolic foam products such as insulation boards are first manufactured, thermal conductivity (λ value) at 23° C. is typically 0.017-0.024 W/m·K depending on the blowing agent selected. Such low thermal conductivity values are typical of a closed cellular structure which retains the blowing agent, and thus are indicative of substantially fewer cellular defects. Cell size is typically 30-200 μm. For effective insulation, foam products, including laminated foam products such as boards, are required to have low thermal conductivity stability (λ value) for a long time. To prove long-term low thermal conductivity stability at room temperature, samples of foam products such as boards can be thermally aged, for example at 70° C. or 110° C. for an extended time period following the procedures in European Standard EN 13166:2008 (or EN 14314:2009). If the λ value is low and stable after such accelerated thermal ageing, it is then reasonable to assume that the foam products such as the insulation boards, that show such low and stable values will provide long-term low thermal conductivity in service.
In the manufacture of acid cured phenolic foam, the manufacturing conditions used must be carefully controlled if a closed cell structure is to be achieved. If stringent procedures are not followed, initial λ values can be as high as 0.030 to 0.040 W/m·K for 25 to 60 kg/m3 density foam; indicating loss of closed cell integrity and ingress of air into the cells. The type and amount of catalyst used in phenolic foam manufacture has a profound effect on the long-term stability of the foam cells. Increased acid catalyst levels tend to result in foam with high initial λ values, and/or foam in which the λ value increases unacceptably with time.
There is a requirement in the construction industry for a phenolic insulation foam that shows not only good initial thermal conductivity properties but also retains those properties when aged. This means that the product not only exhibits good insulative properties when first installed but also exhibits relatively good insulative properties over its lifetime, which may be many decades.
Many attempts have been made to impart good aged insulative properties to foams. These include adding various surfactants to improve foam foaming and foam stability, plasticisers to impart flexibility to the foam thus avoiding cracking within the foam, utilising different types of filler, varying the blowing agent utilised, varying process parameters such as temperature and degree of catalysis. Despite this, some commercially available phenolic foams sold for the construction market, heating and ventilation applications, and industrial purposes do not show good long term retention of thermal conductivity.
On the other hand other properties of foams or the resins used to form them have been addressed. For example UK Patent No. GB1351476 deals with a number of problems. It is firstly concerned with the physical properties of the uncured resin and in particular producing resin compositions that are flowable so that they can be easily pumped into cavities between walls and cured in-situ. Fillers are said to impede the flow of the foam into the hollow spaces. To address this issue the composition is formulated as an aqueous foamable phenolic resin that comprises a phenol-aldehyde condensate, a particulate mineral filler, “waterglass” and an expanding agent. This patent is concerned with providing resins that show good flowability so that the resin can be introduced into the space between cavity walls and also providing resins with non-flammable properties. Accordingly its teaching is about having a formulation that on the one hand does not have too much filler so that flowable properties are satisfactory, and on the other hand compensating for any reduction in filler, as reducing the amount of filler deleteriously affects the flammability properties of the material formed from the compositions.
“Waterglass” is defined in the patent as being sodium and/or potassium silicate. The “EXAMPLE” and the “COMPARATIVE TEST” appears to show that the waterglass content has the “surprising effect” of increasing flame resistance. The waterglass is thus used as a water-based inert filler that is substituted for particulate filler within the composition in order to strike the balance between the flowability of the resin and the resultant, flammability properties of the cured foam.
The present invention is concerned with a different problem, that is, achieving low thermal conductivity and in particular achieving low long term aged thermal conductivity (as defined in EN 13166:2008 or EN 14314:2009).
French Patent Publication FR 2,157,674 describes adding sodium silicate to phenol. The sodium silicate de-protonates the phenol in an acid base reaction with consumption of the sodium silicate.
US 2003/0216847 describes a closed-cell foam made from a novel cross-linked novolac-epoxy resin to which, in common with GB1351476 discussed above, sodium silicate is added as a flame retardant material.